The analysis of exhaled breath is an important monitoring tool in modern hospital settings. Through the analysis of fluid mechanical properties such as flow and volume, information about pulmonary functions can be extracted. As the lungs are the location where the gasses are exchanged between blood and air, the difference of major air constituents such as O2, CO2 and H2O between inhaled and exhaled breath are indicative of the arterial blood gases. Also the blood diffusion of anesthetic agents can be followed through breath analysis. Finally, trace markers such as NO can relate on pathologies within the lungs or airway.
The measurement of CO2 in exhaled breath is known as capnography. Carbon dioxide is produced in the body through aerobic metabolism. It is then transported by blood flow to the heart and then the lungs, before being exhaled. If the patient is on a respirator, the CO2 continues along a respiratory pathway to the respirator. En route or at the respirator the level of CO2 is measured. The CO2 is removed and O2 is supplied in the cycle back to the patient's respiratory system. The oxygen is absorbed by the lungs into the blood. The blood is pumped by the heart, thereby transporting the oxygen throughout the body. The cycle continues as cells of the body produce CO2 which is then transported by blood flow.
Capnography measurements are particularly important in emergency and surgical procedures and for long-term respiratory assistance.
It is needed to identify correct positioning of an endotracheal tube or laryngeal mask. Failure to detect a faulty positioning can very serious or fatal.
The ASA (American Society of Anesthesiologists) recommends capnography for every patient receiving general anesthesia, and, more generally, for continual monitoring and the identifying of correct positioning of tubes or masks with respect to the patient in establishing a cyclical respiratory pathway.
The level of CO2 in exhaled breath is an indicator that helps in diagnosing hypoxia, i.e., insufficient oxygen in the blood, so that countermeasures can expeditiously be taken before the medical subject suffers irreversible brain damage. Hypoxia can occur when, for example, a conscious but sedated patient becomes over-sedated and slips into unconsciousness that results in respiratory obstruction. In such a situation, respiratory obstruction can be detected early via capnography, whereas hypoxia (which is detectable through pulse oximetry) occurs considerably later, when the time left to remedy the situation is short.
Capnography, can, for instance, also detect circulatory failure, e.g., cardiac arrest. If blood is not delivered to the lungs, the level of CO2 in exhaled breath drops. This can be detected early through capnography, so that resuscitation can be commenced.
CO2 is also the main contributor to the pH level in the blood. The body regulates the breathing rate according to the pH level.
Other major constituents of air are oxygen, water vapor and nitrogen. Oxygen levels, like carbon dioxide levels, have clinical significance, but can be adequately monitored on the input (i.e., inspiration) side. Under some circumstances, water vapor levels also assume clinical significance, as with the asthmatic patient.
Other gases encountered in exhaled breath include ethanol, ingested through a liquid substance, and anesthetic vapors, the anesthetic having been administered to prepare for surgery and being supplied continuously at lower levels during the procedure.
Capnography can be done in either of two distinct measurement configurations, namely the mainstream and the sidestream configuration. In the mainstream configuration, the gas sensor device is placed on the tube that goes into the airway of the patient. It measures the entire flow and has a fast response. It does require, though, that an endotracheal tube is been placed in the patient. The sidestream configuration on the other hand uses a small tube that extracts at a continuous pace some air from the airway from the patient. That air then goes via a sample line to an offside module where the sensor is placed. Because of the gas transport, the sidestream configuration has a slower response than the mainstream configuration, yet by placing the tube right at the exit of the nose and mouth, sidestream provides a less invasive technique. However, since the introduction of very low volume sample lines, the lower time resolution associated with sidestream is no longer valid.
The first analysis of gases with ultrasound goes back to the 1920's, when both quartz ultrasound transducers and stable frequencies became largely available.
The detection technology generally used for CO2 detection (capnography) is Non-Dispersive Infra Red absorption (NDIR) at 4.3 nanometers (nm) (i.e., the wavelength absorbed by CO2). It is perceived as being the only technology that meets the (60-100 ms) time-resolution and specificity demands Chemical detection is also possible and generally cheaper, but it brings classically longer time constants along which prevent time-resolution demands from being met.
A difficulty with ultrasound lies in the ability of a solid to transfer its movement to a gaseous medium. This transfer is generally highly inefficient in terms of energy, because of an impedance mismatch between the ultrasound transducer and the gaseous medium.
Piezoelectric transducers are commonly used to transmit and receive ultrasound, but face this inefficiency if the medium is gaseous.
The capacitive micromachined ultrasonic transducer (cMUT) provides a better impedance match to fluid media.
Researchers at Bilkent University have studied using cMUTs in fluid media for air-borne applications. “Stagger tuned cMUT array for wideband airborne application,” Selim Olcum et al., 2006 IEEE Ultrasonics Symposium, pg. 2377. They found that since the center frequency of a cMUT element depends on its radial size, a 60% bandwidth in air is possible when different sized cMUTs are connected in parallel. Using more transducer elements with different cell radii further increases the bandwidth, but the study does not indicate by how much.